populate_sdk_ext: set noexec flag for some tasks

populate_sdk_ext: set noexec flag for some tasks

For the SDK, what we are interested in is the sysroot part. So the
only useful task is do_populate_sysroot. Other tasks like do_package
don't need to run. So mark them as 'noexec' for SDK.

This would result in two benefits:
1) The size of extensible SDK is larged reduced. For example, extensible
   SDK for core-image-minimal is reduced from 1.8G to 793M.
2) The installation time is reduced as a lot of tasks are not run.

[YOCTO #7590]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>